1. Field of the Disclosure
The present invention relates in general to disk drives and, in particular, to a system, method and apparatus for a fluid dynamic bearing for a hard disk drive.
2. Description of the Related Art
Hard disk drives (HDD) typically utilize a bearing upon which one or more magnetic media disks is supported and rotated. One type of bearing is known as a tied-shaft fluid dynamic bearing (FDB). These types of designs are usually employed for structural rigidity in heavy-loaded HDDs, such as those HDDs with two or more disks, or HDDs with large form factors.
Two types of tied-shaft FDB designs are currently popular. FIG. 1 depicts a conical-shaped design and FIG. 2 depicts a spool type design. For the conical shaped FDB 11, both the top and bottom cone surfaces 13, 15 have a large curvature. When the rotating parts (i.e., the bearing sleeves 17, 19) contact the stationary parts (i.e., the cones 13, 15, respectively) during motor spin down, the parts contact each other along a line instead of along their surfaces. This design reduces wear of the parts over a lifetime of numerous starts-stops during HDD operation. The manufacturing tolerance, such as the parallelism of the cones and bearing sleeves, may have a lesser impact on bearing wear since the curvature of the cone surface reduces the possibility of the surface contact.
In contrast, the spool type design 21 in FIG. 2 has a thrust bearing 23 (i.e., the rotating part) that always makes surface contact with the thrust bush 25 (i.e., the stationary part). Moreover, during assembly, the lower thrust bush 27 is always either loose fit or tight fit to the shaft 29 to a prescribed location to form the bottom thrust bearing. After assembly, the bottom thrust bearing tolerance control of the perpendicularity is limited by the loose or tight fit process. To avoid thrust bearing seizure during contact between the stationary and rotating parts under an unexpected external force, the material used to form the bottom bearing bush material is often a relative soft material, such as bronze or brass. The top thrust bush is not affected since it can be made as a single item. The perpendicularity of the shaft and thrust bush can be better controlled than the loose or tight fit assembly of the lower thrust bearing.
Moreover, the spool type tied-shaft FDB design is available only for a 2.5 inch form factor HDD. For a heavier-loaded 3.5 inch form factor, the reliability of the FDB is substantially unacceptable due to excessive wear between the bottom thrust bearing and bush. Accordingly, improvements in heavy-loaded FDB design continue to be of interest.